[vorbis-dev] Thought for the new year

Gregory Maxwell greg at linuxpower.cx
Wed Dec 27 17:14:26 PST 2000

On Thu, Dec 28, 2000 at 01:57:27AM +0100, Segher Boessenkool wrote:
> I disagree. Look at what a MDCT does to an attack... It gets a very
> flat spectrum. That's no good. You want smaller (effecetive) windows
> at higher frequencies to adjust to the highly dynamic range of sound.
> For video, ou don't care that much, as in the presence of bright
> pixels, the dim pixels will be invisible; in audio, this is true only
> _sometimes_.

But what does an attack 'sound' like? 
A rigid block size mdct is not the perfect transform for human perception,
but I don't think it's that bad. An adaptive spectrogram with suitable
backend processing would be better, but are FAR too slow for our purposes.

I think the greater problem is that when you quantize the mdct of a sample
with transients, the result sounds more dissimilar then you would expect
from the level of quantization, mostly due to de-localization.

Ideally you would analyze the signal in a multiscale frequency power over time
space (ideally an adaptive windowed spectrogram), but compress the audio
using a transform that does better with respect to localization. 

> > Ideally what you want to model is the human perceptual response to signal.
> > All we need to do is take a living human ear, and the appropriate 'chunk of
> > brain', plug it's output back into the computer to create a 'human ear
> > transform'. :)
> That would be great, as you would get _very_ low bitrate; but the
> problem would be the inverse transform :-(

Not really, if you were able to do this, you could create an approximate
set of transforms. I think what you would find is that one ear+brain differ
too much from another and after sufficient generalizing you wouldn't be much
better off then we are now. :)

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